(a) Field of the Invention
This invention relates to a method for continuously monitoring the quality of zinc sulfate electrolyte containing Sb (V)and, more particularly, to a method for measuring the cathode polarization potential of zinc deposition and controlling the electrolyte purification and electrowinning processes in response to deviations of recorded values of the potential from the desired values.
In the process for electrowinning zinc from zinc sulfate solutions, impurities such as antimony, germanium, copper, nickel, cobalt, iron, cadmium and lead, when present above certain critical concentrations, cause re-solution of deposited zinc and a corresponding decrease in the current efficiency of the zinc deposition. To reduce the concentration of impurities in electrolyte, a complex purification procedure, which generally includes an iron oxide precipitation and a zinc dust treatment, is employed prior to electrolysis, In addition to the purification, polarizing additives such as glue may be added to the electrolyte to reduce the effects of the remaining impurities and to provide smooth and level deposits.
(b) Description of the Prior Art
The prior art contains a number of references related to methods for determining the effects of impurities, glue and other addition agents on electrodeposition processes for metals, and for determining the purity of zinc sulfate solutions.
Such methods are disclosed by C. L. Mantell et al (Trans. Met. Soc. of AIME, 236, 718-725, May 1966), H. S. Jennings et al (Metallurgical Transactions, 4, 921-926, April 1973), O. Vennesland et al (Acta Chem. Scand., 27, 3, 846-850, 1973), T. N. Anderson et al (Metallurgical Transactions B, 7B, 333-338, September 1976), M. Maja et al (J. Electrochem. Soc., 118d, 9, 1538-1540, 1971) and P. Benvenuti et al (La Metallurgia Italiana, 60, 5, 417-423, 1968), and in Can. Pat. No. 988,879 and U.S. Pat. Nos. 3,925,168, 4,146,437, 4,132,605, 4,406,753, 4,217,189, 4,324,621, and 4,443,301.
Although a number of methods disclosed in the prior art make it possible to continuously monitor and control the quality of electrolytes by means of a moving cathode, the sensitivity of these methods can be seriously affected under certain conditions. More specifically, when applied to zinc sulfate electrolyte used for the electrowinning of zinc, the sensitivity of the method is decreased or lost when the electrolyte contains antimony in the pentavalent state. The presence of Sb (V) in the electrowinning process is due to the high oxidation potential of the electrolyte which contains typically Mn.sup.3+ and MnO.sub.4.sup.- ions. Sb (V) can also be present in certain materials occurring in the process for the recovery of zinc and used in the preparation of electrolyte which results in the presence of Sb (V) in the purification process.